Direct thermal paper is a thermosensitive recording material on which print or a design is obtained without an ink ribbon by the application of heat energy thereto. Direct thermal paper comprises a base sheet, a base coating and a thermosensitive coating with color forming chemicals that respond to heat.
The most common type of thermosensitive coating used on direct paper is the dye-developing type system. This typically comprises a colorless dye (color former), a bisphenol or an acidic material (color developer) and sensitizer. These solid materials are reduced to very small particles by grinding and incorporated into a coating formulation along with any optional additives such as pigments, binders and lubricants. The coating formulation is then applied to the surface of a support system, typically a base sheet and base coating. The color is formed by application of heat to the thermosensitive coating to melt and interact the three color producing materials.
Thermal printing on thermosensitive recording materials provides a number of advantages over printing on plain paper using inked ribbons. One advantage is that thermal printers are less noisy than impact printers. With fewer mechanical operations, thermal printers are believed to be more reliable than impact printers. There are some compromises which must be made when switching from bond paper to thermal paper because the color producing components require special handling and conditions.
To replace plain paper receipt rolls, it is often desirable that the thermal paper also provides security features and preprinted information such as store logos, advertisements, rules and regulations, etc. It is also desirable that this preprinted indicia be of high quality.
By adding features to thermal paper, care must be taken not to pre-react the reactive components within the thermosensitive coating of the thermal paper or prevent the formation of an image on the thermal paper when passed through a thermal printer. Certain chemical factors can adversely affect and degrade the performance of the thermosensitive coatings and should be avoided such as some organic solvents, plasticizers, amines and certain oils.
The use of ink with optically variable compounds as a security measure is well known. Optically variable compounds change color or reflect a unique wavelength in response to a change in ambient conditions such as exposure to a light source other than ambient light or a change in ambient temperature. Optically variable compounds as defined herein include fluorescent compounds and photochromic compounds which respond to infrared or ultraviolet light, thermochromic compounds which change color at different temperatures and near infrared fluorescent (NIRF) compounds which reflect radiation in the near-infrared range. Examples of fluorescent compounds include those described in U.S. Pat. Nos. 4,153,593, 4,328,332 and 4,150,997. Examples of thermochromic compounds are described in U.S. Pat. Nos. 4,425,161; 5,427,415; 5,500,040; 5,583,223; 5,595,955; 5,690,857; 5,826,915; 6,048,347; and 6,060,428. Examples of near infra-red compounds (NIRF) include those described in U.S. Pat. Nos. 5,292,855; 5,423,432 and 5,336,714. The use of fluorescent compounds as a security feature for thermosensitive recording materials is described in U.S. Pat. No. 5,883,043. The use of NIRF compounds as a security feature for thermosensitive recording materials is described in U.S. Pat. No. 6,060,426, assigned to the assignee as the present invention.
To protect thermal paper from environment conditions, and premature coloration from handling, a number of developments have been made. One is to produce a barrier or protection on top of the thermal coating as disclosed in U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,44,819; 4,507,669 and 4,551,738. A U.V. cured silicone acrylate/methacrylate protective coating for a thermosensitive layer is described in U.S. Pat. No. 4,604,635.
U.S. Pat. No. 5,595,955 discloses coating a latent image comprising a thermochromic ink on the reverse side of thermal paper with a thin protective layer.